A device of this type, for example a steam generator for a fast breeder nuclear reactor cooled by liquid sodium, comprises an elongate cylindrical casing disposed with its axis vertical, closed top and bottom with convex ends.
In such a steam generator, liquid sodium is fed to the upper part of the casing by nozzles traversing this casing and emerging in a space which is bounded by a central sleeve which is coaxial with the steam generator casing and by an annular perforated plate connected to the casing and to the central sleeve and disposed at the lower part of the latter. The central sleeve extends a certain distance above and below the liquid metal entry nozzles, the annular plate fastened to the lower part of this central sleeve thus being below the nozzles and constituting the lower part of the liquid sodium overflow.
Within the space into which the nozzles emerge, and which constitutes the liquid sodium overflow, deflectors are arranged which enable the liquid sodium to be distributed over the whole of the annular plate.
In the case where the liquid sodium is fed by two diametrically opposed nozzles, these deflectors constitute two assemblies each arranged opposite a nozzle and each providing the distribution of the liquid sodium over an annular half-plate.
These deflectors comprise horizontal plates arranged opposite each of the nozzles and occupying the major part of the space which exists between the central sleeve and the casing.
Above the nozzles the casing is lined with a sleeve coaxial with the casing and of slightly less diameter than the casing, for thermal protection of the upper part of the latter.
The liquid sodium fills the whole of the casing up to a level located above the nozzles, i.e., in the zone of the casing which is protected by the peripheral sleeve which includes openings allowing the passage of a certain quantity of sodium between the casing and the peripheral sleeve.
The space inside the casing from the level of liquid sodium to the upper end of the casing contains an inert gas such as argon.
The zone into which the nozzles emerge and which acts as the liquid sodium overflow is separated from the part of the casing situated above the nozzles by a solid annular plate fastened to the casing and to the central sleeve.
The part of the casing situated below the liquid sodium overflow encloses a nest of tubes wound around a body arranged axially inside the steam generator casing. The liquid sodium circulates in contact with the exterior surface of the tubes of the nest inside which the feed water flows that evaporates as a result of heat exchange with the circulating liquid sodium.
At the base of the nest, the liquid sodium passes into the central hollow body, which is perforated with holes, to be evacuated to the lower part of the steam generator casing. Collectors disposed at the input and output of the nest of tubes allow for the distribution of feed water and the recovery of the steam produced by heat exchange inside the nest.
A device of this type, comprising a perforated distribution plate and an assembly of horizontal plate deflectors, makes it possible both to ensure as uniform as possible a circulation of liquid sodium towards the base and to suppress turbulence in the upper level of sodium. In fact, it is necessary to avoid such turbulence in the upper level of the liquid sodium, which is liable to create thermal shocks and to induce the generation of gas.
Moreover, in the event of a leak from an element of the nest of tubes leading to a reaction between the liquid sodium and the water circulating in the nest, the pressure wave resulting from this very energetic reaction must be able to move freely towards the top of the casing of the steam generator to expand itself at the free surface of the liquid sodium.
In prior art devices, the pressure wave passes along the chimney constituted by the central sleeve whose upper limit is a little below the level of the liquid sodium, but in reality this chimney does not present an adequate passage for the pressure wave.
The pressure wave therefore propagates in the liquid sodium overflow situated at the periphery of the central chimney, is reflected by the deflecting plates disposed in this overflow, and propagates through the liquid sodium entry nozzles towards the primary sodium/secondary sodium exchangers, disposed in the nuclear reactor, which ensure the heating of the secondary sodium that constitutes the liquid metal in the steam generator, by the primary sodium, filling the nuclear reactor tank, and constituting the primary fluid. These sodium/sodium intermediate exchangers may therefore be damaged by the pressure wave.